1. Field of the Invention
The present invention relates to a turbomachine compressor such as a turbojet engine or an airplane turboprop engine, comprising at least one stator stage formed by variable-pitch blades.
2. Description of the Related Art
A stator stage of a compressor of this type comprises an annular row of variable-pitch stator blades which are carried by an external casing of the compressor. Each blade comprises an airfoil section which is connected at its radially external end to a radial cylindrical pivot by a mounting plate having an approximately circular contour, said pivot defining the rotational axis of the blade and being guided in rotation in a corresponding orifice in the external casing. The radially internal end of the airfoil section of each blade generally comprises a second cylindrical pivot extending along the rotational axis of the blade and guided in rotation in an orifice in an internal casing of the compressor. In a known manner, the radially external end of the external pivot of each blade is connected by a link rod to a control ring which is rotated about the external casing by a ram or the like. The rotation of the control ring is transmitted by the link rods to the external pivots of the blades and causes them to turn about their axes.
The angular pitch of the stator blades in a turbomachine is intended to adapt the geometry of the compressor to its operating point and in particular to optimize the efficiency and the surge margin of this turbomachine and to reduce its fuel consumption in the various flight configurations.
Each of these blades can be moved in rotation about its axis between a first “open” or “wide open” position, in which each blade extends approximately parallel to the longitudinal axis of the turbomachine, and a second “closed” or “almost closed” position, in which the blades are inclined with respect to the axis of the turbomachine and thus reduce the air flow cross section through the blade stage. When the blades are in the open position, the flow of air flowing through the compressor is at a maximum and when the blades are in the closed position, the flow of air flowing through the compressor is at a minimum (for given operating conditions). The blades can assume intermediate positions between these two extreme positions in order thus to adapt to the variations in the flow of air circulating in the compressor.
When the turbomachine is running at low speed or at idle speed, the variable-pitch blades are brought into their closed position and when the turbomachine is running in full throttle mode (for taking off for example), the blades are brought into their open position.
At low speed, despite the closed position of the stator blades, the angle of incidence between the flow direction of the air in the duct of the compressor and the profile can reach high values which give rise to air separations, further reducing the air flow cross section through the blade stage. These separation regions are mainly located at the radially internal and external ends and disappear when the incidence of the fluid on the blades reaches lower values.
It is known to limit these separations in cascades of stator blades by air bleeds in the regions in question, by virtue of orifices formed in the external or internal casing in the environment of the blades. However, the geometry of these bleeds is generally fixed and, although the bleed is beneficial to given operating conditions of the compressor, its continuous presence, including under conditions where it is not necessary, can affect the performance of the engine (by degrading the efficiency of the compressor and thus the specific consumption). In addition, a fixed bleed geometry limits the optimization capacities of the surge margin of a compressor.